Filament composite tubes typically include aligned reinforcement fibers in combination with thermosetting or thermoplastic resin. Such tubes are manufactured by the application of resin-impregnated reinforcement fibers or rovings to an internal cylindrical mandrel. The rovings are applied under controlled tension in precise orientations and thicknesses to produce a tube wall with desired properties. This can be accomplished by filament winding, whereby the mandrel is rotated about its centerline and the roving is applied along the mandrel by a carriage assembly. Braiding can also be used, whereby the internal mandrel is passed through roving delivery heads which orbit around the mandrel. Numerous hybrid processes exist which combine features of filament winding and braiding.
Commonly used reinforcement fiber materials are carbon, aramid and glass formulations. The reinforcement fibers may include axial reinforcements or circumferential (hoop) reinforcements. The axial reinforcements are sized to provide the tube with the axial strength and/or stiffness required for a particular application. The circumferential or hoop reinforcements are sized to provide the tube with the circumferential strength and/or stiffness required for a particular application.
The resin stabilizes and transfers load between the reinforcements fibers and protects the fibers from environmental attack. Thermosetting resins such as epoxies, phenolics, vinyl esters and polyesters are most commonly used. Less commonly used are thermoplastics such as nylons. The thermosetting resins are used most commonly because they can be applied to the roving in liquid form, which aids in the removal of entrapped air or volatiles. The resin is solidified by the addition of heat energy, resulting in a rigid fiber-reinforced structure. The internal mandrel then is withdrawn, typically to be used again.
For applications requiring the containment of liquid or gases under pressure, an elastomeric or thermoplastic liner typically is used inside the tube to prevent migration of the contained fluids through the composite wall.
Composite tubes as described above are used in a variety of product applications, including oil and gas production and development applications, including tubing, casing and risers. Such tubes are also representative of a pressure vessel with a large port opening relative to its cylinder diameter, a common configuration for rocket motor cases. They also are used as a structural composite strut or link applicable to light-weight truss or frame systems. In these applications, a joint is required to react primarily to axial loadings, resulting from applied axial tension and/or internal pressure. Joints are required between tube lengths in order to afford good service in these applications. The joints typically are provided by end fittings at wound-in interfaces between the tubes and the fittings. The end fittings are generally hollow, rigid structures typically fabricated of metallic or like material.
The wound-in interface between a filament composite tube and a rigid interior end fitting often includes one or more "traplock" grooves in the exterior of the fitting and into which the filament or reinforcements of the composite tube are wound and/or compacted.
In such a traplock joint, the axial load is transferred between the composite tube and the end fitting through bearing on the inboard or load-carrying face of the traplock groove. The surface area of the load-carrying face is one of the parameters determining the strength of the joint or interface. The bearing area can be increased by increasing the height of the load-carrying face. However, the bearing stress which the composite material can support is relatively low (30 to 50 ksi). The diametral envelope required by a single traplock groove can become quite large as the height of the load-carrying face is increased. The diametral requirements of the joint can be reduced by the use of multiple traplock grooves, but there is no efficient method known for determining the precise number of grooves necessary and such determinations typically are arbitrary. In addition, the use of more than one traplock groove does not necessarily result in improved joint performance. It is desirable that all traplock grooves be equally reinforced and all traplock grooves carry an equal share of the load. If the interface is not designed properly, the load may not be distributed equally between the multiple traplock grooves. It is possible to load any one traplock groove to failure before other traplock grooves carry significant load.
Still further problems are encountered in designing such traplock interfaces because a stiffness mismatch generally exists between the composite tube and the rigid end fitting which often is of metallic material. This stiffness mismatch tends to concentrate the greatest share of the load on either the furthest inboard or the furthest outboard traplock groove, depending on whether the end fitting or the composite tube has higher sectional stiffness. If the fitting is stiffer than the composite tube, this difference in sectional properties sometimes can be lessened by the addition of localized reinforcements in the composite tube. However, this solution can become quite expensive and can result in a large joint diameter relative to the tube body.
Still further problems are encountered in establishing and maintaining a pressure-tight seal between the composite tube and the end fitting. This is particularly true if the composite tube has an interior liner. A pressure-actuated (O-ring) seal, for instance, is not feasible because the fitting is encapsulated in the end of the composite tube during its fabrication. The flow of the resin prior to its consolidation precludes the forming in place of sealing features such as grooves or glands. Reliance on an adhesive bond between the end fitting and the tube liner is not reliable due to the differential movement in the axial direction that is inherent in traplock system operation. As the fitting moves outboard under load, the liner material and adhesive typically cannot accommodate this differential movement without cracking, tearing or unbonding.
The present invention is directed to solving the above myriad of problems by providing features at the interface between a filament composite tube and a rigid end fitting to enhance the properties and performance of the components at the interface.